1. Field of The Invention
The present invention relates to a voltage measuring apparatus or an apparatus for the measurement of electrical voltages, and particularly to an apparatus for detecting and measuring infinitesimal voltages existing on the local parts of measured objects.
2. Related Background Art
There are methods for measuring voltages using an E-O probe of electro-optic materials, such as a method for the measurement of infinitesimal voltage signals given on the local parts of measured objects by detecting voltage changes on the local parts. More particularly, there is a method to detect infinitesimal voltages given on the local parts of measured objects, that makes use of a change of a refractive index of the electro-optic materials caused by a change of the electrical field established in the E-O probe disposed in the proximity of the local part of the measured object. See, U.S. Pat. No. 4,446,425.
An example of an existing voltage measuring system using an E-O probe is shown in FIG. 10. A refractive index of the E-O probe 4 varies in response to the electrical field established on the measured device 2, such as, a semiconductor device. A light beam emitted from a light source 6, which is controlled by a driving apparatus 5, passes through a collimating lens 8, a polarizing beam splitter 10, a wave plate 12 and a focusing lens 14 in this order and enters the E-O probe 4. Only the component of the light beam reflected by and returned from the E-O probe 4 that is orthogonal to the incident beam is separated by the polarizing beam splitter 10 and enter the photo detector 16 where the component light beam is converted to an electric signal. The electric signal reflects the change in the electrical field formed on the E-O probe 4, that is, the change in the electric signal given to the local part of the measured device 2.
FIG. 11 is a curve showing the operation principle of the voltage measuring apparatus in FIG. 10. In a case where the measured device 2 is one in which a circuit is formed with stripes on the surface of a substrate and an adequate signal source is connected with the device, the wave plate 12 is supposed to be removed. As indicated by the curve, the relationship between the voltage V given to the measured device 2 and the output intensity I detected by the photo detector 16 is generally described by the formula I=I.sub.0 sin.sup.2 {(.pi./2) (V/V.sub..pi.)}. The output intensity I varies from I.sub.1 to I.sub.2 as the voltage V varies from V.sub.1 to V.sub.2. Accordingly, the voltage change in the circuit of the measured device 2 which the E-O probe 4 is in the proximity of, that is, the change in an electric signal given by an outside signal source to the part of the circuit can be known measuring the output of the photo detector 16 with an adequate measuring instrument. The wave plate functions to give a bias voltage optically, and the output intensity I should be I.sub.0 /2 with a wave plate, when voltage V is not given. The output intensity I is varying from I.sub.0 /2 when a voltage is given.
However, a sufficient detecting sensitivity could not be attained with the existing apparatus for measuring voltage because the structure is formed to detect a minute change of refraction caused in the E-O probe 4 by detecting the change in status of polarization of the light entering and returned from the E-O probe.